CemTec is a magic Runeword created by combining a Cem Rune of Ritual and a Tec Rune of Invocation.
How to Create CemTec
You can create CemTec by adding the magic Cem Rune of Ritual and the magic Tec Rune of Invocation to a two-socketed Helm, Chest Armor, Pants, or Two-Handed Weapon.
Cem - Cast Evade. (50)
Tec - Evoke the Barbarian's Earthquake, dealing damage to enemies within. (100)
Overflow
CemTec can not overflow because the amount of Offering you receive from the Cem Rune exactly fills the required Offering of the Tec Rune.
Overflow occurs when your Ritual generates more Offering than required for the Invocation. The overflow effect is proportional to the amount Offering overflows.